In recent years, according to a portable telephone, a wireless device or the like, various wireless systems are mounted to one apparatus to be able to carry out wireless communication at any time and at anywhere. Generally, a wireless frequency allocated to a wireless system differs for respective wireless systems. Therefore, a wireless device dealing with a plurality of wireless systems is mounted with a plurality of pieces of antennas operated in accordance with frequencies allocated to the respective wireless systems, or a wide band antenna operable in accordance with a plurality of frequencies.
However, small-sized formation of a wireless device is progressed and it is difficult for a wireless device having a plurality of pieces of antennas to sufficiently maintain a distance between the antennas. Therefore, a problem that an isolation characteristic between the antennas is deteriorated is posed.
It is disclosed by, for example JP-A-2006-42111 (pages 2 through 6, FIG. 1), that an isolation characteristic between antennas is improved by restraining a current flowing at a base plate.
According to the antenna disclosed in JP-A-2006-42111, an isolation characteristic between antennas A, B is improved by providing a non power feed element in a linear shape constituting one wavelength of an operating frequency of an antenna by a loop path length including a base plate between the antennas A and B arranged at one side of the base plate.
This is because a current flowing at the non power feed element and a current flowing from the antenna A to the antenna B constitute phases inverse to each other between a substrate and a portion of the non power feed element connected thereto to cancel by each other, and therefore, the current flowing from the antenna A to the antenna B can be reduced.
However, according to a technique disclosed in JP-A-2006-42111, the loop path length of the non power feed element includes the base plate constitutes 1 wavelength of the operating frequency, and a current flowing at the main plate flows to the non power feed element and the non power feed element is resonated. When the loop of one wavelength formed by the non power feed element including the base plate is resonated, the antenna A and the non power feed element as well as the antenna B and the non power feed element are respectively coupled, as a result, the antenna element A and the antenna element B are coupled. Accordingly, it is difficult to improve an isolation characteristic between the antenna A and the antenna B.
Further, the non power feed element radiates a radio wave by resonance, and therefore, there poses a problem that radiation characteristics of the antennas A and B are deteriorated. Further, the loop path length needs to be as long as one wavelength. The non power feed element is enlarged, and it is difficult to mount a small-sized antenna apparatus.